1. Field of the Invention
This invention relates to a method for detecting a focusing error and an optical head using this method, and more particularly to a method of detecting a focusing error which can easily adjust the position of an optical component and an optical head suitable for use in optical information processing apparatus such as an optical disk device, an optical card device, or an optical tape device.
2. Description of the Prior Art
Most of conventional focusing error detecting methods adopted for the optical heads of the optical disk devices or the like utilize the fact that the shape or the light intensity distribution of the reflected light from the disk varies according to a focusing error of the light. In this case, the reflected ray from the disk is received by a multi-division phot-detector, and an unbalance in the DC-like output signals is used as a focusing error detection signal.
For example, an astigmatic method is disclosed in JP-B-54-41883 (first prior art). If the reflected beam from the disk is given astigmatism by being passed through an astigmatic element such as a cylindrical lens, the reflected beam from the disk is focused in two focal lines perpendicular to each other, and the beam profile is circular of a position of the minimum confusion circle roughly midway between the two focal lines. This is the reason way a four-division photo-detector is placed at this minimum confusion circle position to receive the reflected beam from the disk. The shape of the reflected beam formed on the detection surface of the four-division photo-detector is substantially circular when the disk is located at the focal point. If the disk is displaced from the focal point, the shape of the reflected beam becomes two lines perpendicular to each other according to the direction in which the disk is displaced from the focal point. Thus, DC-like output signals from the phot-detector elements of diagonal position of the four-division photo-detector are added separately. A difference between two DC-like addition signals is taken as a focusing error detection signal.
On the other hand, besides the above-mentioned method of detecting an unbalance of DC-like output signals obtained by receiving changes in the shape or light intensity distribution of the reflected light from the disk by a multi-division photo-detector, another method is proposed in JP-A-1-303632 (second prior art) for detecting a focusing error by using a diffraction grating which forms light spots at different positions in the direction of the focus depth.
According to embodiment disclosed in JP-A-1-303632, the diffraction grating used constituted a part of a group of multiple concentric circular grating grooves, the interval of which increase or decrease gradually, and gives aberrations of positive and negative longitudinal shifts of an image point (aberration of focus) to the plus and minus first-order beams. This diffraction grating emits the plus and minus first-order beams in the opposite directions across the principal light axis of the main beam by using a region where the concentric circular grooves are decentered from the principal light axis of the main beam. Therefore, the plus and minus first-order beam are converged by an objective lens as two side spots at different positions in the direction of the focus depth either respect to the main beam. The quantities of reflected rays from the two side spots are modulated by signals recorded on the optical disk, and the degrees of modulation are detected by photo-detecting elements and an envelope detection circuit. The degrees of modulation by the two side spots change with the forcusing error at the optical disk, so that a difference is found between the two degrees of modulation to provide a focusing error detection signal.